Hydraulic cement compositions and methods of making and using the same

ABSTRACT

A hydraulic cement comprising a calcium silicate and at least one phosphate compound. The phosphate compound is included in an amount sufficient to react a major portion of the calcium hydroxide that is produced during hydration of the cement to hydroxyapatite or other calcium phosphates. The phosphate compound is preferably a mono-calcium phosphate. The cement is useful in both bio-medical/dental and engineering applications. The calcium hydroxide is reacted by the phosphate to form hydroxiapatite or other calcium phosphate that is co-precipitated with the calcium silicate hydrate to form a composite-like structure on a nano-scale level. The reduced calcium hydroxide content in the set cement increases its strength and reduces its pH. The hydroxiapatite content and the reduced pH render the cement bio-active and suitable for use in medical and dental implants, for example, for replacement bone and tooth material. Due to its high strength, the cement may also be used for structural/engineering applications.

RELATED APPLICATIONS

This is a continuation application of patent application Ser. No.11/390,702, which was filed Mar. 27, 2006, now U.S. Pat. No. 7,575,628,issued Aug. 18, 2009, entitled “Hydraulic Cement Compositions andMethods of Making and Using the Same”, which claims the benefit ofprovisional patent application Ser. No. 60/664,977, which was filed Mar.25, 2005.

BACKGROUND

1. Field of the Invention

The present invention relates to high-strength hydraulic cementcompositions, and, more particularly, to high-strength hydraulic cementcompositions comprising phosphate compounds and calcium silicates, andmethods for making and using the same.

2. Related Art

Hydraulic cements have very long history, but many aspects of theirchemistry have yet to be completely understood.

Thousands of years ago, Pozzolana, a volcanic ash from Mount Vesuvius,was mixed with limestone, to prepare a powder that hardened (“set”) whenmixed water. This pozzolanic (natural) cement was later reproducedartificially, by heat treating of a mixture of limestone, clay, andbauxite at temperatures in excess of 1500 C. The resulting cement“clinker” was crushed and mixed with gypsum and other additives toresult in ordinary Portland cement (OPC). The process is largelyunchanged today, although minor changes in composition and variousadditives have been introduced to achieve particular properties. Currentglobal production of OPC approaches 1T for every person on earth eachyear, consuming vast amounts of resources and energy; thus anymodification which would decrease the use of resources, whilemaintaining or improving the properties of cement, could have asignificant impact on the environment and our civilization.

There are two main compounds in Portland cement: dicalcium silicate(C2S, also known as Alite) and tricalcium silicate (C3S, also known asBelite). Highly crystalline calcium hydroxide (Ca(OH)₂) (referred toherein as CH) and amorphous calcium-silicate-hydrate (referred to hereinas C—S—H) are formed in the hydration of these two principal components(C2S and C3S). The hydrated cement paste consists of approximately 70%C—S—H, 20% Ca(OH)₂, 7% sulfoaluminate, and 3% secondary phases. Majorproblems caused by the calcium hydroxide, which is formed as a result ofthe setting reaction, are that CH is soluble in water and has lowstrength, which properties negatively affect the quality of concrete; aswill be discussed below, the present invention decreases the finalcontent of CH in the set cement, thus resulting in a significantlyincreased durability and strength.

General purpose Portland type cement (ASTM I) typically containsapproximately 50% C3S, 25% C2S, 12% C3A (tricalcium aluminate3CaO.Al₂O₃), 8% C4 AF (tetracalcium aluminoferrite 4CaO.Al₂O₃.Fe₂O₃),and 5% calcium oxide CaO. The total amount of calcium silicates (C3S andC2S) is approximately 75%, with the predominant silicate being C3S.

A number of investigators have reported achieving an improvement in themechanical strength of Portland cement, by adding silica fume (SiO₂,referred to as S) in order to decrease calcium hydroxide content in thehydrated cement (Mitchell, et al, “Interaction of silica fume withcalcium hydroxide solutions and hydrated cement pastes”, Cement andConcrete Research (1998), 28(11), 1571-1584 and Persson “Seven-yearstudy on the effect of silica fume in concrete” Advanced Cement BasedMaterials (1998), 7(3/4), 139-155). The mechanism of improvement dependson the silica fume reacting with calcium hydroxide to produce anamorphous C—S—H gel with a high density and low Ca/Si ratio. Therefore,no new phases are introduced to the set cement. This demonstrates thatremoval of CH from the set cement provides a substantial improvement inquality. The present invention discloses an alternative method ofin-situ removal of CH from setting cement, by reactively precipitatingcalcium phosphates, in particular hydroxyapatite; this provides not onlyenhanced strength and degradation resistance but also enhancedbiological properties.

Phosphate-based hydraulic structural cements ares well known (e.g., seeFriedman et al “BoneSource hydroxyapatite cement: a novel biomaterialfor craniofacial skeletal tissue engineering and reconstruction” Journalof Biomedical Materials Research (1998), 43(4), 428-432). However, thesecements do not contain silicate material. Strength development andhardening of these cements during setting does not rely on hydration ofcalcium silicates, and does not involve precipitation of C—S—H gel andCH.

Ma et al (“Effect of phosphate additions on the hydration of Portlandcement” Advances in Cement Research (1994), 6(21), 1-12) reported theeffect of phosphate additions on the hydration process of Portlandcement. The reaction products were amorphous, but hydrothermal treatmentat 160° C. of ordinary Portland cement (OPC) modified by CaHPO₄ allowedtransformation of a poorly crystalline phosphate phase intohydroxyapatite. Generally, the presence of sodium and calcium phosphatesresulted in improved flexural strengths. However, a number ofdisadvantages limit the usefulness of the process disclosed by Ma et al,such as the need for hydrothermal treatment to form hydroxyapatite andthe need for high pressure (28 MPa) pressing in order to prepare sampleshaving adequate strength. Also, the process described by Ma et al cannotbe used to form a uniform composite structure, and the mechanicalstrength of the phosphate-modified samples was not significantlyimproved by comparison with Ordinary Portland cement

Chemically bonded ceramics (CBC), in the system CaO—SiO₂—P₂O₅—H₂O, wereinvestigated by Hu et al (“Investigation of hydration phases in thesystem CaO—SiO ₂ —P ₂ O ₅ —H ₂ O” J. Mater. Res. 1988, 3(4) 772-78) andSterinke et al (Development of chemically bonded ceramics in the systemCaO—SiO ₂ —P ₂ O ₅ —H ₂ O” Cement and Concrete Res. 1991 (21)66-72). CBCpowders were synthesized by a sol-gel process and then fired at atemperature of 700-1000 C. for 2 hours. The components of the powdersbefore hydration are calcium hydroxyapatite (major), calcium silicatehydrate, γ-2CaO.SiO₂, amorphous calcium silicate, and amorphous calciumphosphate (Hu, et al, “Studies of strength mechanism in newly developedchemically bonded ceramics in the system CaO—SiO ₂ —P ₂ O ₅ —H ₂ O”Cement and Concrete Res. 1988 (18)103-108). However, the mechanicalproperties were not improved when the samples were hydrated at roomtemperature. Also, since the hydroxyapatite phase precipitated beforehydration of the cement (i.e., it did not participate in the cementhydration), it did not reinforce the cement and thus did not contributeto its increased strength. In order to increase the mechanical strengthof CBC, the samples were formed under high pressure (345 MPa) and werehydrated at high temperature.

Recently, Portland cement based materials (referred to as mineraltrioxide aggregate, MTA) have been used for dental applications, such asendodontic dental treatment (Vargas et al., “A Comparison of the Invitro Retentive Strength of Glass-Ionomer Cement, Zinc-Phosphate Cement,and Mineral Trioxide Aggregate for the Retention of Prefabricated Postsin Bovine Incisors” J. Endodont. 30(11) 2004, 775-777). MTA consistsprimarily of tricalcium silicate, tricalcium oxide and silicate oxide.(Torabinejad et al. “Physical and chemical properties of a new root-endfilling material”. J Endodont 21 (1995) 349-253). It is used in manysurgical and non-surgical applications, and possesses thebiocompatibility and sealing abilities requisite for a perforationmaterial (Lee, et al, “Sealing ability of a mineral trioxide aggregatefor repair of lateral root perforations” J Endod 1993; 19:541-4.). Itcan be used both as a nonabsorbable barrier and restorative material forrepairing root perforations. Because it is hydrophilic and requiresmoisture to set, MTA is the barrier of choice when there is potentialfor moisture contamination, or when there are restrictions in technicalaccess and visibility.

The physical and chemical properties of MTA have been tested and theinitial pH on mixing was 10.2 rising to 12.5 after 3 hours. The MTA wasdemonstrated to be significantly less toxic than other root-end filingmaterials when freshly mixed, and toxicity was negligible when fully setat 24 h (Mitchell, et al, “Osteoblast biocompatibility of mineraltrioxide aggregate” Biomaterials 20 (1999) 167-173) it also has goodcompressive strength after setting.

Torabinejad et al (U.S. Pat. No. 5,415,547 and U.S. Pat. No. 5,769,638)disclosed an improved method for filling and sealing tooth cavitiesusing an MTA cement composition. The cement composition resemblesPortland cement, and formed an effective seal against re-entrance ofinfectious organisms. However, the cement was gray in color, which isunsuitable for most dental applications. Moreover, although the MTAcement has been demonstrated to be non-toxic towards living tissue, itcontains aluminum which is not well accepted by living tissue ifreleased in ionic form. The hydration product of calcium aluminates area mixture of calcium-sulfate-aluminate compounds (Concerte, J. F. Young,pp 76-98, Prentice-Hall, Inc, Englewood Cliffs, 1981). In permanent andlong term implants, such as dental fillings, bone implants, andorthopedic surgery, the calcium sulfate aluminates will continuallyrelease aluminum ions into the human biological system (Fridland, etal., “MTA Solubility: A Long Term Study”, JOE—Volume 31, Number 5, May2005, and Journal of Endodontics, Vol. 29, No. 12, December 2003).Considerable literature indicates that aluminum ions are toxic to humanbiological systems. For example, aluminum directly inhibitsmineralization of bone or is toxic to the osteoblast. Diseases that havebeen associated with aluminum include dialysis dementia, renalosteodystrophy and Alzheimer's disease; aluminum also has an effect onred blood cells, parathyroid glands and chromosomes.

Recently, a white form of MTA (which is substantially iron-free) hasbeen released, which addressed the concerns related tocolor-compatibility for dental applications. However, the modified“white” MTA is still essentially OPC with aluminum as one of itscomponents. Primus (US Pat. Appl. No. 20030159618) disclosed a processfor making a white, substantially non-iron containing dental materialformed from Portland cement. The material still contains aluminum in itschemical composition. Moreover, while this process decreases the ironcontent it does not improve the biological properties of thesematerials, because it does not include any calcium phosphate phases andin particular does not include hydroxyapatite.

Hydraulic calcium phosphate cements (CPC) are another material widelyused for variety of bio-medical applications. CPC was first reported ina binary system containing tetracalcium phosphate (TTCP) and dicalciumphosphate anhydrate (DCPA) (L. C. Chow et al. J. Dent Res., 63, 200,1984). The CPC advantages include self-setting (similar to OPC), butadditionally it includes an apatitic phase in the set cement (e.g. HAP)Consequently, CPC is a bio-active material actively interacting withbody fluids through dissolution-reprecipitation process. This has led toapplications such as bone replacement and reconstruction, and also drugdelivery devices (M. Dairra, et al. Biomaterials, 19 1523-1527, 1998; M.Otsuka, et al. J. of Controlled Release 43 (1997)115-122, 1997; Y.Tabata, PSTT, Vol. 3, No. 3, 80-89, 2000; M. Otsuka, et al. J. of Pharm.Sci. Vol. 83, No. 5, 1994). Research into CPC is quite active, and twoinventors in the present matter were also co-inventors in U.S. Pat. No.6,730,324 which disclosed a novel process for CPC and its applications.

CPC is typically formulated as a mixture of solid and liquid componentsin pertinent proportions; which react to form the apatite HAP. Thephysicochemical reactions that occur upon mixing of the solid and liquidcomponents are complex, but dissolution and precipitation are themechanisms primarily responsible for the final apatite formation (C.Hamanish et al J. Biomed. Mat. Res., Vol. 32, 383-389, 1996; E. Ferandezet al J. Mater. Sci. Med. 10, 223-230, 1999). The reaction pathway inmost CPC systems does not lead to stoichiometric HAP, but rather tocalcium-deficient Ca_(10-x)(HPO₄)_(6-x)(PO₄)_(6-x)(OH)_(2-x), similar tothat found in bone. The major drawback of CPC technology is lowmechanical strength (generally below 20 MPa compressive), whichextremely limits its applications for medical materials and devices.

Silica also enhances the bioactive properties of materials. Acombination of the oxides of calcium, phosphorous and silicon in properproportions (with a majority of silica, of about 45 wt %) results in awell known bioactive glass material, with excellent in-vivo performanceand stimulation of cell growth (e.g. Oonishi et al, “ParticulateBioglass compared with hydroxyapatite as a bone graft substitute”, J.Clin. Orthop. Rel. Res. 334, 316-25, 1997; also U.S. Pat. No. 5,811,302by Ducheyne et al, Sep. 22, 1988). Unfortunately, although chemicallyadvantageous, bio-glass must be processed at very high temperatures(generally in excess of 1000 C.), and is rather a dense, weak andbrittle material. Another disadvantage of bio-glass is that it does noteasily dissolve in biological environment (due to dense SiO₂ filmcoverage), which is desirable in some applications, e.g., forstimulation of bone growth.

The literature has reported recent attempts to address these issues, bycombining the three oxides of calcium, phosphorous and silicon intoporous crystalline composite material, which would possess highbioactivity similar to the bio-glass, but which would be stronger (eventhough porous) and easier to resorb in-vivo (A. R. El-Ghannam, “Advancedbioceramics composite for bone tissue engineering: design principles andstructure-bioactivity relationship”, J. Biomed. Mater. Res. 69A,490-501, 2004). The precursors to the three oxides (plus sodium oxide)were heat treated at high temperatures (130-800 C) to result in a porouscomposite of crystalline silica and variety of calcium-phosphates orcalcium-sodium-phosphates. Excellent bioactivity of these composites wasdemonstrated. Unfortunately the need for the high temperature treatmentmakes this composite material difficult to use as biomaterial, as allthe processing and shaping operations must take place outside of theapplication/implantation site.

Accordingly, there exists a need for a hydraulic cement having improvedproperties, in particular high early strengths, high overall compressivestrength, rapid setting time, low hydration heat, resistance todegradation, and good expansiveness to offset shrinkage. Still further,there exists a need for such a hydraulic cement that can be readilymodified to have biocompatible and bioactive properties, so as to beuseful for medical and dental applications, as well as engineeringapplications.

SUMMARY OF INVENTION

The present invention discloses new high strength bioactive hydrauliccement calcium phosphate silicate cement (CPSC) compositions and methodsof making them and using them in variety of engineering andnon-engineering applications. Due to the unique combination ofproperties of CPSC, examples include uses in such widely diversifiedfields as construction and medical/dental materials and devices. It willbe understood that those skilled in the relevant arts may identifynumerous other uses for the cement of the present invention, even if notspecifically indicated in the present disclosure.

The CPSC cements of the present invention have relatively highmechanical strength (e.g. in comparison to OPC or CPC), relativelyhigh-stability against corrosive environments, and adjustable settingand hardening times. In contrast to previously compositions, the CPSCprovided by the present invention is self-setting at room temperatureand pressure, with higher early strength and low hydration heat. Inaddition to the above properties, which are valuable in structuralapplications of CPSC, the cements contain significant amounts of calciumphosphate (particularly hydroxyapatite) and are therefore highlybioactive, highly biocompatible, and extremely durable in variety ofbiological environments.

The hydraulic cement compositions comprise at least one phosphatecompound and at least one calcium silicate compound. Variants includecompositions similar to Ordinary Portland Cement (OPC), modified throughaddition of phosphate compounds. Other variants are calcium-silicatecontaining cements unlike OPC compositions.

In a preferred embodiment, the main components of the CPSC cement are(i) calcium oxide (CaO) in the range of about 45%-80% by weight of thecement composition, preferably 55 wt %-70 wt %, (ii) silica (SiO₂) inrange of 10%-35% by weight preferably 15 wt %-30 wt %, and (iii)phosphate (in the form of P₂O₅ or alternative ionic form) in range of1%-30% by weight preferably 3 wt %-15 wt %.

Complex chemical and physical reactions and processes take place afterthe hydraulic CPSC cement powder components are mixed with water. Thesereactions involve hydration of calcium silicate compounds and thedissolution of phosphate compounds, and co-precipitation of calciumphosphates, such as hydroxyapatite and calcium silicate hydrate gel. Akey discovery is that the dissolution of phosphate compounds andprecipitation of calcium phosphates takes place during, and utilizes theby-products of, the hydration of calcium silicate compounds, such ascalcium hydroxide, to further precipitate additional calcium phosphatecompounds. Additionally, the co-precipitating calcium silicate hydrategel consumes water available from the decomposition of phosphateprecursors to the hydroxyapatite. It is therefore believed that thesetting process of CPSC is a new and complex phenomenon discovered forthe Ca—P—Si—O—H system.

Another aspect of the present invention is directed to high strengthcement compositions obtained through modification of Ordinary Portlandcement (OPC) by addition of phosphate ions. The inorganic chemicalphosphate modifier, preferably monocalcium phosphate. reacts in-situwith the calcium hydroxide (CH) that forms during hydration of the PCcomponents (i.e. mainly di-calcium silicate C2S and tri-calcium silicateC3S), thus removing the CH which is the structurally weak component inthe body of set cement. Reaction of CH with the phosphates leads toprecipitation of calcium phosphates, in particular hydroxyapatite (HAP).Such Calcium Phosphate Silicate Cement (CPSC) has enhancedfunctionality, in particular enhanced corrosion resistance (due toabsence of CH) and enhanced biocompatibility and bioactivity (due to thepresence of HAP).

Therefore, in one embodiment OPC is modified to produce CPSC. Duringsetting of such cement the phosphate ions react with the excess calciumhydroxide (CH) resulting from the hydration of the principal componentsof OPC, i.e., di-calcium silicate (2CaO.SiO₂ or C2S) and tri-calciumsilicates (3CaO.SiO₂ or C3S). Ordinarily, in the absence of thephosphate ions, the CH forms inclusions of variable size and shape inthe principal gel (CaO—SiO₂—H₂O gel, also expressed as C—S—H) structureof the set PC. These CH inclusions are weak spots structurally andchemically, i.e. they do not contribute strength but rather weaken OPC,and additionally are sensitive to environmental effects (up to 25 wt %of such CH may accumulate within the body of un-modified OPC).

The present invention reacts phosphate ions with the CH that is producedduring OPC hydration, to form variety of calcium phosphate inclusions,the most stable and strongest being hydroxyapatite (HAP). The HAPinclusions in CPSC contribute substantially to the overall compressivestrength of the set cement, both directly (through bonding to the C—S—Hstructure) and indirectly (through removal of the structurally weak CHinclusions). Additionally, HAP is much more resistant to environmentaleffects than CH, rendering CPSC more corrosion resistant that OPC.Additionally, the presence of HAP or other phosphate inclusions withinthe cement structure brings enhanced bio-compatibility and bio-activity.It should be noted that it is not possible to achieve a similarcombination of properties by mechanical mixing of OPC (or similarcement) with the hydroxyapatite or other calcium phosphates; this isbecause in CPSC the two principal components (C—S—H and HAP)co-precipitate (in the composition of the present invention) to form aunique microstructure having a high-degree of homogeneity on a level ofapproximately 100 nm.

Since the CPSC is on average twice as strong in compression as comparedto non-modified ordinary Portland Cement (OPC), one embodiment of theinvention provides a method for using the CPSC hydraulic cementcompositions for structural applications. In these types of applicationsthe high mechanical strength, adjustable setting time, and stability toagainst the corrosions and heat are of primary importance. Consequently,the CPSC cement compositions of present invention can be used for makinghigh strength mortar, concrete, and other materials common in theconstruction industry.

Furthermore, since the CPSC converts a substantial amount of the CHresulting from hydration of the calcium silicates into bio-compatibleand bio-active calcium phosphates, the present invention also providesmethods for the use of CPSC for medical devices, such as prostheses,implants, and other surgical procedures. As noted above, the calciumphosphate, (particularly hydroxyapatite) renders the cement bioactive,biocompatible, and durable; in addition, the removal of CH through HAPformation lowers the pH of the mix, which makes CPSC more compatiblewith the living tissue. These characteristics also make the cementparticularly useful for dental cements, such as dental root-end fillingmaterial, retrofilling materials, pulp capping, apexification, and forsealing of perforations.

These and other features and advantages of the present invention will bemore fully understood from a reading of the following detaileddescription with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating compressive strength of Portland cementmodified to produce CPSC in accordance with the present invention,showing the compressive strength as a function of the calcium phosphatecontent of the cement;

FIG. 2 is a graph of FTIR spectra of OPC and of cement compositions inaccordance with the present invention having varying percentages ofcalcium phosphate;

FIG. 3 is a set of electron microscope scans providing side-by-sidecomparison of the results of bio-activity tests conducted for ordinaryPortland cement and a CPSC cement in accordance with the presentinvention having a calcium phosphate content of about 20%, at 5,000×magnification;

FIG. 4 is a set of electron microscope scans providing side-by-sidecomparison of the microstructures of the fracture surfaces of ordinaryPortland cement and of a CPSC cement in accordance with the presentinvention, at 2,000× magnification;

FIG. 5 is a set of electron microscope scans providing side-by-sidecomparison of the microstructures of the fracture surfaces ordinaryPortland cement and a CPSC cement in accordance with the presentinvention, at 5,000× magnification; and

FIG. 6 is a graph of x-ray defraction patterns obtained for ordinaryPortland cement, for CPSC formed by modification of Portland cement inaccordance with the present invention, and for pure calcium hydroxideand pure hydroxyapatite as references.

DETAILED DESCRIPTION

The present invention provides novel cement compositions and methods ofmaking them, and for using them in variety of application fields,ranging from engineering and construction uses to medical/dentalimplants and fillings. The novel Calcium Phosphate Silicate Cement(CPSC) of present invention has relatively high early strength andrelatively high overall compressive strength (as compared to OPC settingat similar conditions for the same time), adjustable setting time, lowhydration heat, resistance to degradation, high bioactivity andbiocompatibility, and stability to against corrosive environments.

The CPSC is obtained through a chemical process of in-situ formation(co-precipitation) of hydroxyapatite/calcium silicate hydrate gelcomposite at room- or nearly room-temperature and pressure, accompaniedby the removal of CH during cement hydration. This is accomplished byreacting the CH in-situ with phosphate ions to precipitate much strongerand chemically resistant calcium phosphate, in particular hydroxyapatite(HAP), that is intimately mixed with the C—S—H gel. The resultingcomposite cement has high mechanical strength, but alsobiocompatibility, bioactivity, and adjustable setting time. Theseproperties do not require application of hydrothermal treatment orpressure-assisted forming of the components.

The major components of the CPSC cement compositions of the presentinvention, which normally make up approximately 60% by weight of cementin the cement composition, are at least one calcium silicate compoundand at least one phosphate compound. Suitable calcium silicate compoundsinclude, but are not limited to, dicalcium silicate C2S (2CaO.SiO₂),tricalcium silicate C3S (3CaO.SiO₂) and mixtures thereof. Tetracalciumsilicate C4S (4CaO.SiO₂) may also be used. The calcium silicate compoundmay constitute between 10-99% of the cement composition by weight, andis preferably in the range from about 40-80% of the composition byweight.

Suitable phosphate compounds include but are not limited to, calciumphosphates (including mono-calcium phosphate, di-calcium phosphate, andmixtures thereof), magnesium phosphates, sodium phosphates, zincphosphates, aluminum phosphates, iron phosphates, potassium phosphates,nickel phosphates, zirconium phosphates, phosphoric acid,organo-metallic phosphates, and mixtures thereof. Mono-calcium phosphate(calcium phosphate monobasic) is generally preferred. The phosphatesused in present invention may contain hydration water. More complex(pre-reacted) phosphates may also be used. In addition to mono-phosphate(PO₄ ⁻³) compounds di-phosphate (P₂O₇ ⁻⁴) compounds, tri-phosphate(P₃O₁₀ ⁻⁵) compounds, meta-phosphate compounds, and mixtures thereof,may be used). The phosphate compound is included in an amount sufficientto react a major portion of the calcium hydroxide produced duringhydration of the cement to form hydroxyapatite or other calciumphosphates. The phosphate may be included in an amount within the rangefrom about 1-70% by weight of the cement composition, with 5-30% beingpreferred; a range of about 10-15% has been found particularly suitable,especially when using mono-calcium phosphate.

The minor or ancillary components of the cement compositions of thepresent invention may include, but are not limited to, silicon dioxide,tricalcium aluminate (3CaO.Al₂O₃), tetracalcium aluminoferrite(4CaO.Al₂O₃.Fe₂O₃), calcium sulphate, calcium sulfate dihydrate(CaSO₄.2H₂O), and mixtures thereof, which typically make up less thanabout 30% by weight of the cement composition. Also, the cement maycontain a number of impurity oxides which are present in the originalraw materials, which will normally make up less than about 15% by weightof cement composition, including, but not limited to, iron oxides,magnesia (MgO), potassium oxide, sodium oxide, sulfur oxides, carbondioxide, water, and mixtures thereof.

Calcium compounds may be included, including but not limited to, calciumoxide, calcium carbonates, calcium hydroxides, and mixtures thereof.

When water is mixed with the cement compositions, a complicated set ofreactions is initiated. The phosphate and calcium compounds quicklydissolve in water and precipitate to produce new calcium phosphatecompounds, principally hydroxyapatite when the pH is above 7.0. Thereaction speed is adjustable in range from 20 minutes to 2 daysaccording to the needs of the application, through minor changes in thesystem chemistry and precursor morphology.

Initially, the calcium silicates react with water to produce a calciumsilicate hydrate gel (CaO—SiO₂—H₂O gel); however, the rate of thehydration reaction of the calcium silicates is slower than the rate ofthe formation of the hydroxyapatite. Consequently, in this process ofco-precipitating, nano-size particles of calcium silicate hydrate gelfill the voids among the precipitating hydroxyapatite particles.

A key feature of the present invention is the ability for in-situformation through co-precipitation of hydroxyapatite/calcium silicatehydrate gel composite at room temperature, in ordinary prepared cementpaste, without a need for increased pressure or temperature, and inparticular without a need for hydrothermal treatment of the settingcement paste. The formation of C—S—H/HAP composite is accompanied by adecrease of CH content in the set cement (CH being the weakest componentof the set cement, structurally and chemically). The resulting CPSCmaterial exhibits significantly increased mechanical strength, whereinthe calcium phosphate and hydroxyapatite act as a reinforcement phaseand the calcium silicate hydrate gel is a matrix in the compositestructure.

Reactions

The precipitation reaction (A) of calcium phosphate apatite is asfollows:10Ca²⁺+6PO₄ ³⁻+2OH⁻→Ca₁₀(PO₄)₆(OH)₂  (A)where Ca/P ratio is between 1.2 and 2.0.

The hydration reactions (B, C) of calcium silicates can be approximatedas follows:2[3CaO.SiO₂]+6H₂O→3CaO.2SiO₂.3H₂O+3Ca(OH)₂  (B)2[2CaO.SiO₂]+4H₂O→3CaO.2SiO₂.3H₂O+Ca(OH)₂  (C)where the calcium hydroxide CH is the hydration product which ordinarilycontributes to the high alkalinity of the cement. It is widelyrecognized that the calcium silicate hydrate is not a well-definedcompound and formula of (3CaO.2SiO₂.3H₂O) is only an approximatedescription. The ratio of CaO/Si₂O is in between 1.2 and 2.3, whichdepends on water contain, aging time and temperature, and other factors.The high pH=10-12 during hydration, in presence of phosphate ions PO₄ ³⁻increases the precipitation rate of the calcium phosphate, preferablyhydroxyapatite, according to the reaction (A), which in turn decreasesthe overall alkalinity of hydration. Consequently, a new process iscreated wherein we both (i) decrease the alkalinity and CH content insetting cement; and (ii) provide a strong and bio-active HAP phase whichreinforces the composite.

In order to further remove the calcium hydroxide CH during setting ofthe cement, and therefore further enhance its mechanical strength,additional phosphate may be introduced into the cement composition,which will continue to react with calcium hydroxide to formhydroxyapatite. If the calcium phosphate compound is calcium phosphatemonobasic in the cement, the following dynamic chemical reaction takesplace:3Ca(H₂PO₄)₂+7Ca(OH)₂→Ca₁₀(PO₄)₆(OH)₂+12H₂O  (D)

The calcium hydroxide, produced during the hydration reaction of calciumsilicates, reacts relatively rapidly with the phosphate compounds toproduce a new compound, hydroxyapatite HAP. Importantly, the samereaction (D) also provides water, which continues to react with thecalcium silicates. The water supplied through the dynamic reaction (D)is one of the most important factors to control the hydration reactionspeed, and thus the setting time, hardening time, and the finalmechanical strength of the composite CPSC.

To further improve mechanical strength, silica fume, or any other formof silica, e.g. sol-gel derived silica, may be introduced into thecement composition to react with remnant calcium hydroxide to producethe amorphous calcium silicate hydrate gel, and therefore furtherdecrease the CH content and alkalinity of the cement.

The hydration rate of the calcium silicates (reactions B, C) isincreased in the CPSC composition, since the phosphate compounds reactwith calcium hydroxide to produce hydroxyapatite and water (reaction D),thus shifting the equilibrium. Therefore, the setting and hardening timeof CPSC cement is shortened.

The hydration reactions of calcium silicates normally result in a pHover 12 in ordinary cement compositions. In the present invention, thephosphate compounds react with calcium hydroxide and thus neutralize pHof the cement. The calcium hydroxide is therefore only the intermediateproduct of the hydration reaction of calcium silicates in CPSC cement.

In ordinary Portland cement (OPC), the calcium silicate hydrate C—S—H isan amorphous or poorly crystalline material which forms very smallparticles in the of submicron size (less than 1 um in any dimension).C—S—H is the main strength-providing compound in OPC. The calciumhydroxide is a well-crystallized material with a definite stoichiometry,which occupies about 20-25% of the volume of set OPC cement paste. Thecalcium hydroxide precipitates where free space is available and inextreme cases may completely engulf the cement grains. However, calciumhydroxide is mechanically weak and reduces the mechanical strength andchemical resistance of the cement. As noted above, in the presentinvention the calcium hydroxide is only an intermediate product ofhydration reactions B, C, since it reacts with the phosphate compoundsto produce the hydroxyapatite and water according to reactions A, D.Consequently, the decreased content of CH in the set CPSC provides arelative increase in strength and chemical resistance of the cement.

In set CPSC, the calcium silicate hydrate interlocks with thehydroxyapatite or other calcium phosphate, leading to in situ formationof the composite-like structure on a nano-scale level. Thehydroxyapatite is the reinforcement phase and calcium silicate hydrateis the matrix of the composite structure. Both phases provide mechanicalstrength to the composite. In comparison with ordinary Portland cement(OPC), the mechanical strength and corrosion resistance of presentcement CPSC are therefore significantly improved, because weak phasecalcium hydroxide is replaced with the high strength and chemicallystable hydroxyapatite.

One use of the hydraulic CPSC cement of the present invention is thusfor structural applications, because of its (i) relatively highmechanical strength, (ii) adjustable setting time, (iii) chemicalstability, and (iv) low alkalinity. Consequently, the CPSC cementcomposition of the present invention can be used for making highstrength concrete for block making, reservoirs, pre-cast operations,high structural concrete (bridge, high-rise buildings, dams, and nuclearpowder plants), plaster and mortar, and for use in mining operations,and many other structural and non-structural applications. Thesignificantly less alkaline character of CPSC (as compared to OPC) makesit more suitable for use in combination with alkaline-sensitivematerials, such as some metals, polymeric/organic components, andadditives. Many other uses of such high-strength cement and concrete maybe envisaged.

Hydroxyapatite (HAP) is the major inorganic component, and an essentialingredient, of normal human bone and teeth. The compressive strength ofhydroxyapatite (>60 MPa) is much higher than that of calcium hydroxide(<1 MPa). Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) is one of the mostbiocompatible and bioactive ceramics because it is similar to themineral constituents of human bone and teeth. Due to the relativelylarge content of HAP, the CPSC cement of the present invention is notonly bioactive and biocompatible, but also is osteoinductive andosteogenic (encourages bone in-growth).

Another use of the CPSC cement of the present invention is therefore formedical and dental materials and devices, such as prostheses, implants,fillings, coatings, and other surgical procedures. This is because ofhigh content of HAP The CPSC cement is self-setting, injectable, and ofhigh strength, allowing it to be used for both weight and non-weightbearing applications. The cement resists disintegrative washout uponcontact with blood, and injection into the wound is less stressful tothe surrounding tissue because of it being biocompatible with thephysiological environment. The CPSC is not resorbed (dissolved) incontact with living tissue in body fluids.

Bio-medical applications of the CPSC cement of the present inventioninclude, but are not limited to, percutaneous vertebroplasty,craniomaxillofacial surgery applications, ridge augmentation, spinalfusion cage/implant, treatment of radial fractures, treatment oftemporal mandibular, joint disorders, plastic surgery and cosmeticaugmentation, bone graft substitutes, veterinary applications, and drugdelivery. Other examples include biological use of CPSC or CPSC-basedcomposites with drugs or proteins to address specific medical problems;for example microspheres of CPSC may be designed for targeted deliveryof drugs, proteins, DNA, or other medically active species to the areaof interest in the body. Examples of dental applications include, butare not limited to, dental root-end filling material, retrofillingmaterials, pulp capping, apexification, and sealing of perforations.

The CPSC cement of the present invention can also be used for makingcomposite materials, including specific secondary reinforcement phases,such as fibers, aggregates, bioglasses, bioceramics, polymers, andmetals, in variety of morphological forms such as particles, fibers,loops, and others.

In comparison with mineral trioxide aggregate (MTA), the CPSC cement ofthe present invention has higher mechanical strength and is morebioactive and biocompatible. The setting rate of CPSC is adjustable,thus making it more attractive for dental operations. Since thephosphate compound in CPSC cement composition neutralizes the calciumhydroxide and thus lowers the pH from over 12 to around 8-11 during thehydration reaction, CPSC is more friendly to the surrounding tissues.Radiopaque components may be added to render the CPSC radiopaque for thepurposes of dental diagnostics. Suitable radiopaque substances aregenerally selected from heavy metals, oxides of heavy metals, salt ofheavy metals, and mixtures thereof. For example, zirconia (ZrO₂),tantalum oxide (TaO₂), Barium sulfate (BaSO₄), and Bismuth oxide (Bi₂O₃)are radiopaque materials suitable for such applications; other suitablematerial include the heavy metals gold, silver, barium, bismuth andtantalum; mixtures of the foregoing may also be used. The radiopaquesubstance may be included in an amount within the range from about 3-50%by weight of the cement composition, preferably in an amount within therange from about 10-30%.

The presence of the phosphate compounds that consume CH produced in thecalcium silicates hydration reactions also shifts the equilibrium andtherefore affects the overall kinetics of the setting process. Namely,the dynamic chemical reactions of calcium silicates hydration arespeeded up since the phosphate compounds in cement compositions reactwith calcium hydroxide to produce hydroxyapatite and water. Therefore,the setting and hardening time of the cement decreases. These parametersof the setting reaction are affected by the concentration and solubilityof the phosphate compounds added to the regular hydraulic cement, suchas Portland cement. Therefore, a new method for controlling the settingreaction dynamics, and therefore the release heat and shrinkage, isprovided.

It is also important to emphasize that production of the Portland cementis one of the most energy-consuming and polluting (e.g. in terms ofcarbon dioxide CO₂ released) processes known to humanity. As billions oftons of the cement are produced annually, any advance over OPC thatretains or improves its properties (such as strength and corrosionresistance) while replacing part of the cement with other compounds, asdoes the present invention, may significantly decrease the negativeenvironmental impact of cement production.

Test Results

FIG. 1 illustrates compressive strength of a CPSC cement in accordancewith the present invention as a function of calcium phosphate content.By adding 10-15 wt % calcium phosphates into the Portland cement toresult in CPSC cement, the compressive strength of is significantlyincreased, from about 45 MPa to about 103 MPa (for 7 days setting time).

FIG. 2 illustrates FTIR spectra of reference samples of ordinaryPortland cement (OPC) and the hydroxyapatite HAP, and of the CPSC cementcompositions produced through combination of OPC with differentpercentages of calcium phosphate compounds. When water is mixed with theCPSC cement, a complicated set of reactions is initiated. The calciumsilicates react with water to produce a calcium silicate hydrate gel(CaO—SiO₂—H₂O gel, or C—S—H, as commonly represented in literature)which provides the strength, and calcium hydroxide (CH) whichcontributes to the alkalinity of the cement, but is also a weak link ofthe cement. There are three main regions related to the vibrationfrequencies of SO₄, SiO₄, and CO₃ groups in wave number ranges 500-1300cm⁻¹. For the OPC samples, the sulfate absorption bands (i.e. the S-0stretching bands) at 1150-1100 cm in hydrated Portland cement are shownas a shoulder band. The bands at 960 cm⁻¹ and 520 cm⁻¹ are contributedby the Si—O asymmetric stretching and the Si—O out-of-plane bendingvibration of SiO₄ group of calcium silicate hydrate gel. Another band at870 cm⁻¹ is contributed by the vibration of CO₃ group (refer also to theMollah et al, “A Fourier transform infrared spectroscopic investigationof the earlyhydration of Portland cement and the influence of sodiumlignosulfonate” Cement and Concrete Research 30 (2000) 267-273).

For the hydroxyapatite reference sample (HAP), the absorption spectrumof HAP has three main regions related to the vibration frequencies ofthe Off, PO₄ ³⁻ and CO₃ ²⁻ ions (Vaidya, et al, Pressure-inducedcrystalline to amorphous transition in hydroxylapatite, J. Mater. Sci.,32 (1997) 3213-3217). The absorption of the internal stretching of OH inhydroxyapatite is located at 630 cm⁻¹. The vibration of the phosphateion are an asymmetric stretch (1100-1028 cm⁻¹), a degenerate symmetricstretch (960 cm⁻¹), and a double degenerate asymmetric bend (600 cm⁻¹and 560 cm⁻¹). The CO₃ ²⁻ has vibration band at 870 cm⁻¹. This is atypical amorphous or poor crystallinity structure hydroxyapatite.

In comparison with OPC, the frequencies of bands of vibration andliberation modes of various functional groups of hydroxyapatite doappear on FTIR spectra by adding 10 wt % calcium phosphate compounds.The intensity of absorption bands increases with increasing content ofcalcium phosphate compounds. This indicates formation of hydroxyapatitein the CPSC cement composition.

FIG. 3 illustrates the results of bioactivity tests conducted for (A)ordinary Portland cement (OPC) and (B) CPSC cement with approximately20% of the calcium phosphates co-precipitated with C—S—H gel. Allsamples were immersed in SBF (Simulated Body Fluid) solution at 37° C.for 10 days, and then the samples were washed with distilled water anddried for scanning electron microscope (SEM) observations. There is nohydroxyapatite formation observed on surface of ordinary Portland cementOPC by SEM (although traces of other calcium phosphates may be observedunder higher magnification and/or longer exposure to SBF; this weakeffect was also reported for the OPC-like mineral trioxide aggregate(MTA) utilized for dental cements), which indicates that the OPC is notbioactive or only weakly bioactive. A typical hydroxyapatite structurelayer is formed on the surface of CPSC cement indicating that the cementof the present invention has high bioactivity.

FIG. 4 illustrates the microstructures of fracture surfaces of (A)ordinary Portland cement and (B) CPSC, at 2000× magnification. Scan (A)clearly shows relatively large Ca(OH)₂ crystals; the calcium hydroxidedoes not form homogenous crystals in the cement paste, but rather growsinto the free space, such as pores and voids. By contrast, large Ca(OH)₂crystals are absent in the calcium phosphate silicate cement, asillustrated in Scan (B); instead, a composite of HAP/C—S—H can beobserved on this surface. The in-situ formation of HAP/C—S—H compositewas confirmed by elemental mapping through Energy Dispersive X-ray (EDX)analysis.

FIG. 5 illustrates the microstructures of the fracture surfaces of (A)ordinary Portland cement and (B) CPSC at 5000× magnification. Scan (A)shows the crystal structure of Ca(OH)₂ and calcium silicate hydrated gel(CSH). The pure HAP phase and HAP/CSH composite was observed in the CPSCsample as shown by Scan (B) which was confirmed by EDX analysis.

FIG. 6 illustrates X-ray diffraction patterns for the OPC, and forPortland cement with calcium phosphate added so as to form CPSC.Additionally, for reference, the bottom pattern is from pure calciumhydroxide CH and the top pattern is from pure amorphous or poorlycrystallized hydroxyapatite (i.e. the type of HAP formed during thedissolution-precipitation setting reaction of CPSC). The hydroxyapatitediffraction peak at 32.3 degree of 2-Theta angle starts to appear afteradding 10 wt % of the calcium phosphate (CP) precursors into OPC.Simultaneously the relative intensity of the X-ray diffraction peaks ofcalcium hydroxide decrease with the increase of the calcium phosphatecontent in the OPC (i.e. compare the decrease of Ca(OH)₂ peak intensityat about 47.5 Two-Theta). This X-ray diffraction pattern indicates thatthe calcium phosphate additive dissolves in water, reacts with calciumhydroxide (which is the hydration product of calcium silicates of OPC)and subsequently co-precipitates the hydroxyapatite, in intimate mixturewith the co-precipitating C—S—H gel.

EXAMPLES Example 1 Preparation and Properties of Phosphate SilicateCement CPSC

This example used general purpose commercial ordinary Portland typecement OPC (ASTM I) containing approximately 50% C3S, 25% C2S, 12% C3A(tricalcium aluminate 3CaO.Al₂O₃), 8% C4 AF (tetracalcium aluminoferrite4CaO.Al₂O₃.Fe₂O₃), and 5% calcium oxide CaO. 11 grams of dicalciumphosphate was dried in the furnace at 140° C. for 24 hours and thenmixed with 29 grams of the tetracalcium phosphate and with the OPCcement powder (160 grams) in alcohol solution by ball milling for 24hours. The resulting slurry was spray dried. The average particle sizeof the cement powder was about 10 um. The setting time of the CPSCcement thus was about 2 hours, for a water/cement ratio of 0.21. Theaverage compressive strength after 7-day incubation at 37° C. and 100%humidity was 104 MPa, with a standard deviation of 7 MPa, (FIG. 1). TheX-ray diffraction pattern provided in FIG. 6 indicates that the setcement contained about 15% of HAP, and about 8% of Ca(OH)₂.

These results compared favorably with the characteristics of the controlsamples of the OPC material, hydrated under identical conditions. Theaverage compressive strength of the OPC was 45 MPa, with a standarddeviation of 5 MPa, (FIG. 1). The X-ray diffraction pattern in FIG. 6indicates that the set cement contained no HAP, and about 20% ofCa(OH)₂. The scanning electron microscope microstructures shown in FIGS.4, 5A (OPC) and FIGS. 4, 5B (CPSC) clearly indicate the compositecharacter of the CPSC, and the decreased content of CH in the CPSC.

Example 2 Preparation of High Strength CPSC Cement for OrthopedicApplications

In this example the phosphate silicate cement was prepared syntheticallyusing well defined pure chemicals (as opposed to the poorly definedminerals utilized for preparation of typical commercial Portlandcement). The raw materials used for the preparation of CPSC cement werecolloidal silica (50 wt % Ludox, from 3M company) for SiO₂, calciumhydroxide (99.9%, Sigma-Aldrich) for CaO, tetracalcium phosphate(Ca₄(PO₄)₂O), and dicalcium phosphate anhydrate (CaHPO₄.H₂O) (Fisher).Alternatively, and with no effect on the final properties of CPSC, thecolloidal silica may be derived from thermal decomposition of hydratedsilicon alkoxide such as tetra-eth-oxide silicate (TEOS).

The designed composition of the present cement is 65 wt % tricalciumsilicate, 20 wt % dicalcium silicate, 10 wt % tetracalcium phosphate,and 5 wt % dicalcium phosphate. A 200 g batch was prepared by mixing96.32 g of colloidal silica, 160.98 g calcium hydroxide, and 300 gdistilled water in an alumina jar, and ball milling for 24 hours. Theslurry mixture was dried by using a spray dryer, and was then fired in ahigh temperature furnace at 1550° C. for 1 hour to form tricalciumsilicate and dicalcium silicate, followed by natural cooling to roomtemperature. The resulting cement clinker was ground to −325 sieveparticle size (<45 um particle size), with an average particle size ofabout 10 um. 11.25 g of dicalcium phosphate anhydrate was dried in thefurnace at 140° C. for 24 hours and then mixed with 20 g of thetetracalcium phosphate and combined with the fired cement powder (168grams) in alcohol solution by ball milling for 24 hours. The resultingslurry was spray dried. The average particle size of the cement powderwas about 10 um. The setting time was about 4 hours. The averagecompressive strength after 7-day incubation at 37° C. and 100% humiditywas 101 MPa, with a standard deviation of 8 MPa. The control samples ofthe cement without the addition of phosphates, and set under identicalconditions, had average compressive strength 45 MPa, with a standarddeviation of 5 MPa.

The X-ray diffraction pattern provided in FIG. 6 indicates that the setCPSC cement contained about 15% of HAP. This cement paste was injectableand suitable for orthopedic applications.

Example 3 Preparation of CPSC High Strength Gray and White Cements forDental Applications

The following procedure is to prepare a high strength, pure, bioactive,and biocompatible CPSC cement for dental applications. Raw materialsused are colloidal silica (50 wt % Ludox, 3M) for SiO₂, calciumhydroxide (99.9%, Sigma-Aldrich) for CaO, boehmite (AlOOH) for Al₂O₃,iron oxide (Fe₂O₃, 99% Fisher), calcium sulfate dehydrate (CaSO₄.H₂O,99%, Fisher), Ca(OH)₂, and monocalcium phosphate (Ca(H₂PO₄)₂, 99%,Sigma). Alternatively, and with no effect on the final properties ofCPSC, the colloidal silica may be derived from thermal decomposition ofhydrated silicon alkoxide such as tetra-eth-oxide silicate (TEOS).

The designed composition of the cement is 58 wt % tricalcium silicate(3CaO.SiO₂), 11 wt % dicalcium silicate (2CaO.SiO₂), 6 wt % tricalciumaluminate (3CaO.Al₂O₃), 7 wt % tetracalcium aluminoferrite(4CaO.Al₂O₃.Fe₂O₃), 4 wt % calcium sulfate dehydrate (CaSO₄.2H₂O), 4 wt% calcium oxide, and 10 wt % monocalcium phosphate (Ca(H₂PO₄)₂). A 200 gbatch was prepared by mixing 78 g colloidal silica, 156.9 g calciumhydroxide, 10.57 g boehmite, 4.61 g ferrite oxide, and 300 g distilledwater in alumina jar, and ball milled for 24 hours. The slurry mixturewas dried by using a spray dryer, and then was fired in a hightemperature furnace at 1550° C. for 1 hour, and then cooled naturally toroom temperature, followed by grinding to about 10 um average particlesize. 20 g of monocalcium phosphate, 8 g of calcium hydroxide, and 8 gof calcium sulfate dehydrate were mixed with the fired cement powder inalcohol solution by ball milling for 4 hours. The slurry was dried byusing spray dryer.

The setting time of the CPSC cement prepared as described above wasaround 6 hours, for a water/cement ratio of 0.21. The compressivestrength after 7-day incubation at 37° C. and 100% humidity was 110 MPa,with a standard deviation of 7 MPa. The control samples of the cementwithout the addition of phosphates, and set under identical conditions,had on average compressive strength of 48 MPa, with a standard deviationof 4 MPa. The X-ray diffraction pattern provided in FIG. 6 indicatesthat the set CPSC cement contained about 15% of HAP.

This CPSC cement paste was injectable, of gray color, and suitable fordental applications, such as root-end filling material, retrofillingmaterials, pulp capping, apexification, and most importantly the sealingof perforations. For making white color CPSC cement for the specificdental applications requiring color control (e.g. for cosmetic reasons),the process was repeated exactly, except that the iron oxide wasexcluded from the cement composition. The properties of such whitevariant of CPSC were essentially the same as the properties of the grayvariant of the CPSC.

Example 4 In Vitro Testing of Bioactivity of the CPSC Cements

This example compares the bioactivity of ordinary Portland cement (OPC)to that of the CPSC cement with 10% calcium phosphate. Both cementpowders were prepared by the process described in Example 3, for the“gray” cement containing iron oxide, either without phosphates (controlsample, OPC) or with phosphates (CPSC). The paste samples were preparedby mixing the cement powder with distilled water at the ratio ofwater/cement w/c=0.25. The cement paste was filled into cylinder mold of1 inch diameter and 2 inch height. The samples were incubated at 100%humidity and without use of any organic species, and its pH was adjustedto 7.4 with 7.5% NaHCO₃ solution (Li, et al, Apatite formation inducedby silica gel in a simulated body fluid. J Am Ceram Soc 75 (1992), pp.2094-2097). All samples were immersed in the SBF solution at 37° C. for10 days, and then the samples were washed with distilled water and driedfor SEM observations as illustrated in FIGS. 4-5.

No hydroxyapatite formation was observed on the surface of ordinaryPortland cement by SEM, thus indicating that the OPC is not bioactive.However, a hydroxyapatite structure layer was found on the surface ofthe CPSC cement disclosed of the present invention, indicating goodbioactivity, osteoinductivity, and osteogenicity.

Example 5 CPSC Cement Composition with Radio-Opaque Component for DentalApplications

This example illustrates the procedure for making CPSC dental cementwith radio-opaque material. The fired cement powders were prepared asdescribed in Example 3. Zirconia (ZrO₂, Zircoa, USA) was chosen as theradio-opaque material for dental application because zirconia isbiocompatible and used for orthopedic implant devices. Alternatively,and with no effect on the final properties of CPSC, the radio-opaquematerial may be derived from tantalum oxide TaO₂. 80 g of the CPSCcement powder and 20 g of zirconia were mixed with powder/powder mixerfor 20 min. X-ray tests indicated clear visibility of the modified CPSCcement, demonstrating that the dental cement with zirconia radio-opaqueis suitable for dental applications.

Example 6 Effect of Phosphate Compound on Mechanical Strength

This example further demonstrates that the compressive strength ofPortland cement was significantly improved by adding phosphate compoundsto result in CPSC. The composition of ordinary Type I Portland cementused for the testing was about 50 wt % tricalcium silicate (3CaO.SiO₂),25 wt % dicalcium silicate (2CaO.SiO₂), 12 wt % tricalcium aluminate(3CaO.Al₂O₃), 8 wt % tetracalcium aluminoferrite (4CaO.Al₂O₃.Fe₂O₃), and5 wt % calcium sulfate dehydrate (CaSO₄.2H₂O). The dicalcium phosphateand calcium oxide (weight ratio of DCP/CaO=2:3) was introduced to thePortland cement composition in the amounts of 0 wt %, 10 wt %, 20 wt %,and 40 wt % and mixed in alcohol solution by ball milling for 4 hours,and then dried in oven at 70 C for 12 hours. The mixture of cementpowder was mixed with distilled water at w/c=0.25. The cement paste wasfilled into cylinder molds of 1 inch diameter and 2 inch height. Thesamples were incubated at 100% humidity at room temperature for sevendays. The cylinder samples were used to determine the maximumcompressive strength with an Instron Universal Testing Machine with across-head speed head of 2 mm/min. The samples were furthercharacterized for phase content by SEM and X-ray diffraction. Theresults are illustrated in FIGS. 4, 5, and 6.

Example 7 Process for Making High Strength Concrete with Novel CPSCCement

This example shows that the phosphate modified Portland cement CPSCsignificantly improves the mechanical strength of concrete. The cementsfor testing are ordinary Type I Portland cement and phosphate-modifiedordinary Type I Portland cement as described in Example 6. Coarsecrushed rock (average size: 1.5 inch in diameter) and sand (averagesize: 0.01 inch) were used as aggregates. The compressive testingsamples were prepared by mixing 22 wt % cement powder, 55 wt % coarseaggregates, 18% wt sand, and 5 wt % water. The concrete paste was castinto a mold of 10 cm×10 cm×10 cm by vibration packing method. The castsamples were incubated at 100% humidity at room temperature for 28 daysbefore compressive testing. The samples were tested with an InstronUniversal Testing Machine with a cross-head speed head of 2 mm/min. Theaverage compressive strength of ordinary Portland cement concrete was 84MPa, with the standard deviation of 6 MPa. The compressive strength ofphosphate modified Portland cement CPSC concrete (where CPSC wasprepared as described in Example 1) was 155 MPa, with the standarddeviation of 10 MPa. It indicated that the CPSC cement composition ofthe present invention significantly improved the mechanical strength ofthe concrete.

It is to be recognized that various alterations, modifications, and/oradditions may be introduced into the constructions and arrangements ofparts described above without departing from the spirit or ambit of thepresent invention as defined by the appended claims.

1. A hydraulic cement composition for medical and dental applicationsthat reacts with water at a substantially neutral pH, said hydrauliccement composition comprising: at least one calcium silicate compound;at least one phosphate compound in an amount sufficient to react a majorportion of calcium hydroxide produced during hydration of said cementcomposition to hydroxyapatite or other calcium phosphates so as tomaintain said cement composition at a pH in a range from below pH12 toabove pH7; and at least one alkaline-sensitive organic compound.
 2. Thehydraulic cement composition of claim 1, wherein said at least onecalcium silicate compound is selected from the group consisting of:di-calcium silicate; tri-calcium silicate; tetra-calcium silicate; andmixtures thereof.
 3. The hydraulic cement composition of claim 1,wherein said at least one alkaline-sensitive organic compound comprises:at least one alkaline-sensitive organic polymer compound.
 4. Thehydraulic cement composition of claim 1, wherein said at least onephosphate compound is present in an amount in a range from more than 5%to about 30% by weight of said cement composition.
 5. The hydrauliccement composition of claim 1, wherein said at least one phosphatecompound is selected from the group consisting of: calcium phosphates;magnesium phosphates; sodium phosphates; zinc phosphates; aluminumphosphates; iron phosphates; potassium phosphates; nickel phosphates;zirconium phosphates; phosphoric acid; organo-metallic phosphates; andmixtures thereof.
 6. The hydraulic cement composition of claim 5,comprising said at least one calcium silicate compound in an amountwithin the range from about 40% to about 80% by weight of said cementcomposition.
 7. The hydraulic cement composition of claim 1, whereinsaid pH is in a range from about pH8 to about pH11.
 8. A hydrauliccement composition for medical and dental applications that reacts withwater at a substantially neutral pH, said hydraulic cement compositioncomprising: calcium silicate; at least one phosphate compound in anamount sufficient to react a major portion of calcium hydroxide producedduring hydration of said cement composition to hydroxyapatite or othercalcium phosphates so as to maintain said cement composition at a pH ina range from below pH12 to above pH7; and a medically active additiveselected from the group consisting of: proteins; drugs; DNA; andmixtures thereof.
 9. A hydraulic cement composition for medical anddental applications that reacts with water at a substantially neutralpH, said hydraulic cement composition comprising: calcium silicate; atleast one phosphate compound in an amount sufficient to react a majorportion of calcium hydroxide produced during hydration of said cementcomposition to hydroxyapatite or other calcium phosphates so as tomaintain said cement composition at a pH in a range from below pH12 toabove pH7; and a polymer reinforcement phase in a morphological formselected from the group consisting of: particles; fibers; loops; andcombinations thereof.